Prior to this invention gaseous mixtures, such as air, have been fractionated in heatless fractionating systems by compressing or otherwise pressurizing the gaseous mixture, by passing the compressed gaseous mixture at an elevated pressure through an adsorbent to adsorb one or more components of the mixture, and by thereafter purging the adsorbed components from the adsorbent at reduced pressure. In fractionating air, the adsorbent may be selected to adsorb nitrogen, thus providing an oxygen enriched effluent for use by patients or for other purposes. Fractionating systems which produce oxygen enriched air for breathing are referred to as oxygen concentrators.
In order to establish a continuous operation in which the unadsorbed gas or product effluent is produced in an essentially uninterrupted stream, it has been the practice to use two vessels each containing the desired adsorbent, and to deliver the compressed gaseous mixture alternately to the two vessels to thus alternately pass the compressed mixture through the two bodies or beds of the adsorbent. According to this conventional practice each adsorbent bed is purged of the adsorbed components or constituents during the intervals in which the adsorbent bed is not being used to adsorb one or more components from the compressed gaseous mixture, so that each bed alternately undergoes an adsorption phase at elevated pressure and a desorption phase at reduced pressure. Purging of each adsorbent bed is conventionally accomplished by diverting a fraction of the unadsorbed gas from the bed undergoing its adsorption phase to the bed undergoing its desorption phase. Solenoid operated valves are typically used to control the delivery of the compressed gaseous mixture to the beds and the exhaust of purge gas from the beds.
Examples of the foregoing type of dual adsorbent bed system are described in U.S. Pat. No. 2,944,627 which issued on July 12, 1960, U.S. Pat. No. 3,104,162 which issued on Sept. 17, 1963, U.S. Pat. No. 3,192,686 which issued on July 6, 1965, U.S. Pat. No. 3,225,518 which issued on Dec. 28, 1965, U.S. Pat. No. 3,280,536 which issued on Oct. 25, 1966, U.S. Pat. No. 3,659,399 which issued on May 2, 1972, and U.S. Pat. No. 4,101,298 which issued on July 18, 1978.
One drawback of known, prior art systems of the foregoing type is that at start-up, it takes a significant amount of cyclic operation time to reach the maximum concentration of the key component in the unadsorbed gas or product effluent, as it is called. In prior oxygen concentrators, for example, the cyclic operation time required for reaching maximum oxygen concentration is typically from 10 to 20 minutes.
Another drawback of such dual bed fractionating systems arises from the inlet line pressure which varies from zero psig to a value at least as great as the pressure of the compressed gaseous mixture. To provide satisfactory operation at zero pressure, as well as the elevated line pressures, prior dual bed fractionating systems are customarily equipped with large-orificed, direct lift solenoid operated valves for controlling delivery of the compressed gas to the adsorbent beds. Such valves are usually very large, heavy and expensive. Furthermore, reverse exhaust flow often occurs with the large orifice valves because they are exposed to relatively high back pressures. To alleviate this problem, prior systems may, at added expense, be equipped with additional inlet check valves or a pressure tank.
A further drawback of the prior dual bed fractionating systems using solenoid operated valves is that, upon shutdown or power loss, the compressed gas may be trapped in the adsorbing bed. Such a condition tends to create a back pressure that could, upon subsequent start-up, overload the compressor which is used to compress the inlet air or other gas. Also, such a condition tends to lengthen the cyclic operating time to reach maximum key component concentration when the equipment is subsequently re-started.
Many prior dual bed systems are equipped with timers to control the cyclic operation for the adsorbent beds. Such timer controlled systems are considered to be less satisfactory than pressure controlled systems because the mass of the adsorbed component is affected primarily by the partial pressure and temperature of the adsorbed constituent. However, a pressure controlled system as proposed in the previously mentioned U.S. Pat. No. 4,101,298 is relatively complex.